Arbitrary functions are not serializable with Scala 2.12

[ttim]

Due to the bug in Scala 2.12 https://issues.scala-lang.org/browse/SI-10232 generated arbitrary functions in scalacheck are not serializable. Fix for that is to split ArbitraryArities and GenArities into different traits.

Here is a repro (should be done with scala 2.12):

import org.scalacheck.Arbitrary._
val f = arbitrary[Int => Int]
val outputStream = new java.io.ByteArrayOutputStream()
val objectOutputStream = new java.io.ObjectOutputStream(outputStream)
objectOutputStream.writeObject(f)
objectOutputStream.close
val inputStream = new java.io.ByteArrayInputStream(outputStream.toByteArray)
val objectInputStream = new java.io.ObjectInputStream(inputStream)
objectInputStream.readObject()

Should work but instead cause next exception:

java.io.IOException: unexpected exception type
  at java.io.ObjectStreamClass.throwMiscException(ObjectStreamClass.java:1582)
  at java.io.ObjectStreamClass.invokeReadResolve(ObjectStreamClass.java:1154)
  at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1817)
  at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1353)
  at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:2018)
  at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1942)
  at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1808)
  at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1353)
  at java.io.ObjectInputStream.readObject(ObjectInputStream.java:373)
  ... 29 elided
Caused by: java.lang.reflect.InvocationTargetException: java.lang.BootstrapMethodError: too many bootstrap method arguments
  at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
  at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
  at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
  at java.lang.reflect.Method.invoke(Method.java:498)
  at java.lang.invoke.SerializedLambda.readResolve(SerializedLambda.java:230)
  at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
  at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
  at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
  at java.lang.reflect.Method.invoke(Method.java:498)
  at java.io.ObjectStreamClass.invokeReadResolve(ObjectStreamClass.java:1148)
  ... 36 more
Caused by: java.lang.BootstrapMethodError: too many bootstrap method arguments
  at java.lang.invoke.CallSite.makeSite(CallSite.java:320)
  at java.lang.invoke.MethodHandleNatives.linkCallSiteImpl(MethodHandleNatives.java:307)
  at java.lang.invoke.MethodHandleNatives.linkCallSite(MethodHandleNatives.java:297)
  at org.scalacheck.GenArities.$deserializeLambda$(GenArities.scala)
  ... 46 more

[travisbrown]

I've just confirmed that this issue can be resolved by instantiating FunctionNs explicitly instead of using function literals, which means we don't have to break binary compatibility by splitting up the traits. I'll send a PR in the next half hour or so.

[ttim]

@travisbrown this is awesome, thank you!